Source Port: The port of a source host
Destination Port: The port of a destination host
Length: Total length of header and data
Checksum: Before a packet is sent, TCP runs both its header and data through a specific algorithm to arrive a checksum value, which is placed in the checksum field. When the receiver gets, it runs through the same algorithm. If the value matches, it accepts. If not it drops the packet.
For UDP, it is optional. For TCP, it is mandatory.